1. FIELD OF THE INVENTION
The present invention relates to a knock detecting apparatus adapted for use with internal combustion engine ignition timing control systems, etc., which are designed to function so that knock is detected from the vibration produced outside the engine cylinders due to the cylinder pressure and the ignition timing is controlled so as to obtain the desired knock intensity.
2. DESCRIPTION OF THE PRIOR ART
It is known in the art that there is a close correlation between the ignition timing and the cylinder pressure. When a mixture is burned, in the absence of knocking there is no superposition on the cylinder pressure of any higher harmonic component (usually a frequency component in the range of 5 to 10 kHz which falls in the frequency band determined by the engine cylinder bore diameter and the velocity of sound in the combustion and which is produced by the intermittent and rapid combustion), whereas when knocking occurs such higher harmonics start to superpose on the cylinder pressure at around the maximum cylinder pressure and this results in the generation of vibration or sound outside the cylinder. Examination of the pressure signals generated inside the cylinders and the generation of the vibrations or sound outside the cylinders shows that the beginning of knock (trace knock) starts to occur at an engine crank angle at which the cylinder pressure attains the maximum value and that as the knock is gradually increased (to light knock and heavy knock) the higher harmonic starts to superpose considerably earlier (or on the ignition side) than the maximum cylinder pressure crank angle. Thus, if the vibration or sound produced outside the cylinders by the knock is detected with a high degree of accuracy and then fed back to control the ignition timing, the engine efficiency will be improved greatly. In fact, however, there has been no detecting apparatus which is not only capable of accurately detecting the condition of knocking or a feedback factor but also operable stably under severe surrounding conditions required for vehicles.
A known detecting apparatus of the above type employs a piezoelectric type acceleration sensor to detect the knocking vibration caused in an engine. However, this type of detecting apparatus still has many deficiencies with respect to the following requirements or characteristics which must be met by a knock detecting apparatus for internal combustion engines.
(1) Since the knock in an engine increases in severity as the ignition timing is advanced as mentioned previously, the desired knock control level for ignition timing control purposes must be lower than the trace knock level and it is essential for the apparatus to possess highly accurate and stable detection characteristics. However, the piezoelectric element is essentially a high impedance element and its characteristics tend to be affected by humidity and contamination, thus making it difficult to stably generate a small signal indicative of trace knock (the drift and amplitude variations are high). Further, since the piezoelectric characteristic of the element is provided by polarization and since the working conditions are severe in that they change cyclically from the high to low temperature condition, the polarization tends to loss gradually with the resulting reduction in the sensitivity. The reduced sensitivity results in a rise in the knock control level and this occasionally causes damage to the engine due to the knock.
(2) The detecting apparatus should preferably be low in the manufacturing cost. However, the piezoelectric element has a high input impedance and consequently an expensive amplifier of the high impedance input type which is called as a charge amplifier must be used for amplifying the output signal of the piezoelectric element. The high impedance type amplifier tends to malfunction under the effect of noise caused by the engine ignition noise or the like and consequently the construction must be made more expensive and complicated to overcome this difficulty.
(3) The piezoelectric element is disadvantageous for use as a vehicle-mounted detector from the stand-point of durability, cost, etc., in that the elements is low in shock resistance, tends to be subjected to cracking, breaking, etc., and requires cooling means or the like for using the element at high temperatures.